Multi-tier, multi-RAT (Radio Access Technology) heterogeneous networks (Het-Nets) are a new direction in network architectures for cost-effectively adding cellular capacity and coverage. This architecture comprises a tier of small cells (e.g., picocells, femtocells or relay stations) overlaid on the macro cellular network to augment network capacity. The bulk of the macro network traffic is offloaded to small cells whereas a wide area coverage and mobility is maintained through the macro network. Deployments typically aim for full spectral reuse across the tiers and the cells in the network as licensed spectrum is expensive and scarce. Recent Het-Net architectures also support WiFi-based small cells, exploiting un-licensed spectrum to augment cellular capacity. Multi-RAT (Radio Access Technology) cells integrating both WiFi and Cellular air interfaces in a single infrastructure device are also an emerging trend. When used with multi-RAT client devices, the integrated multi-RAT infrastructure also provides an additional “virtual WiFi” carrier, which can be judiciously exploited to improve capacity and QoS (Quality of Service) performance of multi-tier Het-Net deployments.
In cellular systems, association is typically performed on a per-user basis. Here, each user determines the downlink carrier signal strength and the signal-to-noise-ratio (SNR) corresponding to the base-station, and associates with the base-station with the highest SNR value. While simple to implement, received SNR based methods are not good indicators of the actual throughput a user would experience, since the throughput also depends on the number and mix of users present on each cell in the network. Network coordinated user association and mapping methods can therefore provide better overall performance in terms of actual user “utility” (e.g., user throughput, power efficiency or QoS). One method for performing network assisted association, which has been explored for cellular networks, is based on maximizing “proportional fair” (PF) throughput across users. The PF utility not only depends on the average user throughput (as a function of SNR) but also the load, i.e. number of users, on each base-station. Unlike conventional association, utility-based association cannot be completed on a per-user basis since the user associations are now inter-dependent; changing the association of any one user alters the load (hence the utility) on the serving as well as target base-stations (the old base-station it leaves and new base-station it associates with). As a result, the global utility based association problem is difficult to solve, and sub-optimal heuristic approaches have been proposed as approximate solutions.
Conventional methods of user association may also be used in multi-tier Het-Net deployments where the user can choose a macro base-station (MBS) or a pico base station (PBS) based on received SNR. In this case, user “offload” to pico base stations occurs with no special preference for the PBS. Conventional association methods have a number of limitations in this case as well. Firstly, they fixate the pico coverage range and are therefore dependent on network topology/geometry. For example, in clustered user distributions, users will associate with pico base stations in groups-at-a-time (in a hotspot-fashion) owing to their similar SNR distributions. Secondly, macro base stations are naturally designed for large coverage areas via high transmit power. Conventional offloading may thus lead to over-association with the MBS and under-utilization of the PBSs, particularly in sparse user distributions.
The coverage range of the pico base stations in conventional association can be changed from fixed to dynamic by including an artificial bias value to the SNR reported by the users. Positive bias values effectively assign higher transmit power to the pico base stations hence effectively increasing their coverage. Users are therefore encouraged to associate with pico base station, thus offloading the macro base station. While adding to system-design flexibility, (positive) bias leads to unfavorable interference conditions for the range-extended users (i.e. pico cell-edge users) as they are forced to connect to the weaker (pico) base station even though the signal from the macro user is stronger. The macro base station thus acts as a very strong interferer and effective interference mitigation schemes are required to mitigate the interference from the macro. The 3GPP standard supports Inter-Cell Interference Coordination (ICIC) schemes to manage cross-tier interference in multi-tier HetNet deployments. A typical ICIC approach for managing interference is to “orthogonalize” macro and pico transmissions in the zone of strong interference by creating an interference free zone such as a macro free zone (MFZ), or by blanking macro transmissions on some designated sub-frames (“Almost Blank Sub Frames”-ABS). While this approach improves the overall throughput distribution for most users, it sacrifices precious macro capacity that may also hurt the overall aggregate system throughput.